1. Field of the Invention
The present invention generally relates to methods and apparatus for temperature measurement and control inside a container during a high temperature heating process, and more particularly to the measurement and control of temperatures within a sealed container or retort prior to and during diffusion bonding of a metallic sandwich structure.
2. Background of the Invention
A well-known technique for joining metal is diffusion bonding. This process involves metallurgically joining surfaces of similar or dissimilar metals by applying heat and pressure for a time duration so as to cause co-mingling of atoms at the joint interface.
In the diffusion bonding process, the application of pressure accomplishes a plastic flow of the material to fill normal void areas. If the pressure is too low, small voids will remain at the joint interface and the joint strength will be less than the maximum attainable. The application of pressure also breaks up the surface oxides and surface asperites so as to present clean surfaces for bonding.
The elevated temperatures used for diffusion bonding serve to accelerate diffusion of atoms at the joint interfaces as well as to provide a metal softening which aids in surface deformation thereby allowing more intimate contact for atom bonding and movement across the joint interface.
Through practice of this technique, It has become accepted that diffusion bonding of certain "reactive" metals, such as titanium and its alloys, must be conducted in a controlled environment In order to ensure cleanliness of the materials which are particularly sensitive to oxygen, nitrogen, and water vapor content in the air at elevated temperatures. Unless the metals are protected during this bonding process, they will become embrittled and their structural integrity will be destroyed.
The controlled environment has been attained through the use of a closed, sealed and evacuated metallic envelope known as a "retort". Typically, the retort is provided with external instrumentation for the purpose of monitoring the parameters of the diffusion bonding process. However, it has been found that, under these circumstances, exact measurement inside the retort of the temperatures of the components necessary for successful and repeatable diffusion bonding is not attainable.
Several approaches have been attempted to remedy this problem. At first, analytical modeling was employed using heat transfer calculations. Thereafter, more sophisticated approaches involved instrumenting a dummy part at preselected locations and comparing differences in temperatures between the part itself and locations that were previously monitored. This approach was based on the assumption that these differences were constant, so that, once determined, they would translate into temperature parameters which could be used to control the diffusion bonding process. However, too many extraneous and often unexpected conditions arose which interfered with accurate and repeatable measurements, thereby rendering this technique ineffective.